The present invention relates to a liquid-crystalline medium, to the use thereof for electrooptical purposes, and to displays containing this medium.
Liquid crystals are used, in particular, as dielectrics in display devices since the optical properties of such substances can be affected by an applied voltage. Electrooptical devices based on liquid crystals are extremely well known to those skilled in the art and may be based on various effects. Devices of this type are, for example, cells having dynamic scattering, DAP (deformation of aligned phases) cells, guest/host cells, TN cells having a twisted nematic structure, STN (super-twisted nematiic [sic]) cells, SBE (super-birefringence effect) cells and OMI (optical mode interference) cells. The most common display devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid-crystal materials must have good chemical and thermal stability and good stability toward electrical fields and electromagnetic radiation. Furthermore, the liquid-crystal materials should have low viscosity and give short addressing times, low threshold voltages and high contrast in the cells. Furthermore, they should have a suitable mesophase, for example, for the abovementioned cells, a nematic or cholesteric mesophase, at customary operating temperatures, i.e. in the broadest possible range above and below room temperature. Since liquid crystals are generally used as mixtures of a plurality of components, it is important that the components are readily miscible with one another. Further properties, such as electrical conductivity, dielectric anisotropy and optical anisotropy, must meet various requirements depending on the cell type and the area of application. For example, materials for cells having a twisted nematic structure should have positive dielectric anisotropy and low electrical conductivity.
For example, the media desired for matrix liquid-crystal displays containing integrated nonlinear elements for switching individual image points (MLC displays) are those having high positive dielectric anisotropy, broad nematic phases, relatively low birefringence, very high specific resistance, good UV and temperature stability of the resistance and low vapour pressure.
Matrix liquid-crystal displays of this type are known. Examples of nonlinear elements which can be used to individually switch the individual image points are active elements (i.e. transistors). This is then referred to as an "active matrix", and a differentiation can be made between two types:
1. MOS (Metal Oxide Semiconductor) transistors on a silicon wafer as substrate. PA0 2. Thin-film transistors (TFTs) on a glass plate as substrate. PA0 X: F, Cl, CF.sub.3 or OCHF.sub.2, PA0 Y.sup.1 and Y.sup.2 : in each case H or F PA0 r: 0 or 1. PA0 the rings A.sup.1, A.sup.2 and A.sup.3 are each, independently of one another, 1,4-phenylene, 2- or 3-fluoro-1,4-phenylene, trans-1,4-cyclohexylene or 1,4-cyclohexenylene, PA0 Z.sup.1 and Z.sup.2 are each, independently of one another, --CH.sub.2 CH.sub.2 --, C.dbd.C--, --CO--O--, --O--CO-- or a single bond, and PA0 m is 0, 1 or 2. PA0 Z.sup.0 is a single bond, --CH.sub.2 CH.sub.2 --, ##STR15## proves advantageous for suppressing smectic phases, although this may reduce the specific resistance. In order to achieve parameter combinations which are ideal for the application, a person skilled in the art can easily determine whether and, if yes, in what amount these compounds may be added. Normally, less than 15%, in particular 5-10%, are used. PA0 R.sup.0 is n-alkyl, n-alkenyl, n-alkoxy or n-oxaalkyl, in each case having up to 9 carbon atoms, Y is H or F and X' is CN, halogen, CF.sub.3, OCF.sub.3 or OCHF.sub.2.
The use of monocrystalline silicon as the substrate material limits the display size since even the modular assembly of various part displays results in problems at the Joints.
In the case of the more promising type 2, which is preferred, the electrooptical effect used is usually the TN effect. A differentiation is made between two technologies: TFTs comprising compound semiconductors, such as, for example, CdSe, or TFTs based on polycrystalline or amorphous silicon. Intensive research efforts are being made worldwide in the latter technology. The TFT matrix is applied to the inside of one glass plate of the display, while the inside of the other glass plate carries the transparent counterelectrode. Compared with the size of the image point electrode, the TFT is very small and hardly affects the image at all. This technology can also be extended to fully colour-compatible image displays, where a mosaic of red, green and blue filters is arranged in such a manner that each filter element is located opposite a switchable image element.
The TFT displays usually operate as TN cells with crossed polarizers in transmission and are illuminated from the back.
The term MLC displays here covers any matrix display containing integrated nonlinear elements, i.e. in addition to the active matrix, also displays containing passive elements such as varistors or diodes (MIM=metal-insulator-metal).
MLC displays of this type are particularly suitable for TV applications (for example pocket TV sets) or for high-information displays for computer applications (laptops) and in automobile or aircraft construction. In addition to problems with respect to the angle dependency of the contrast and the switching times, problems result in MLC displays due to inadequate specific resistance of the liquid-crystal mixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September 1984: A 210-288, Matrix LCD Controlled by Double Stage Diode Rings, p. 141 ff., Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Design of Thin Film Transistors for Matrix Adressing [sic] of Television Liquid Crystal Displays, p. 145 ff., Paris]. As the resistance decreases, the contrast of an MLC display worsens and the problem of "after image elimination" may occur. Since the specific resistance of the liquid-crystal mixture generally decreases over the life of an MLC display due to interaction with the internal surfaces of the display, a high (initial) resistance is very important to give acceptable service lives. In particular, in the case of low-voltage mixtures, it was hitherto not possible to achieve very high specific resistances. It is furthermore important that the specific resistance increases as little as possible with increasing temperature and after heating and/or exposure to UV radiation. The MLC displays of the prior art do not satisfy current demands.
It was hitherto possible to prepare liquid-crystalline media with values for birefringence and phase range (e.g. clearing point: &gt;70.degree.) which are required for practical use, but only with threshold voltages of about 1.8 volts, if value is placed on values of about 98% for the holding ratio under extreme conditions (e.g. after exposure to UV).
Thus, there continues to be a great demand for MLC displays of very high specific resistance and at the same time a broad operating temperature range, short switching times and low threshold voltage which do not have these disadvantages or only do so to a lesser extent.
For TN (Schadt-Helfrich) cells, media are desired which facilitate the following advantages in the cells:
broadened nematic phase range (in particular down to low temperatures), PA1 switchability at extremely low temperatures (outdoor use, automobiles, avionics), PA1 increased stability to UV radiation (longer life). PA1 a medium additionally contains one or more compounds selected from the group comprising the general formulae II, III and IV: ##STR4## in which the individual radicals are as defined below: R: alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each case having up to 7 carbon atoms PA1 a medium additionally contains one or more compounds selected from the group comprising the general formulae V to VIII: ##STR5## in which R, r, X, Y.sup.1 and Y.sup.2 are each, independently of one another. PA1 a medium additionally contains one or more compounds selected from the group comprising the general formula [sic] IX to XII: ##STR6## in which R, X, are each, independently of one another. The proportion of compounds of the formulae I to IV together is at least 50% by weight in the total mixture PA1 the proportion of compounds of the formula I is from 10 to 50% by weight in the total mixture PA1 the proportion of compounds of the formulae II to IV is from 30 to 70% by weight in the total mixture ##STR7## the medium contains compounds of the formulae II and III or IV R is straight-chain alkyl or alkenyl having 2 to 7 carbon atoms PA1 the medium essentially comprises compounds of the formulae I to IV PA1 the medium contains further compounds, preferably selected from the following group (Y.dbd.H or F, preferably F): ##STR8## The I: (II+III+IV) weight ratio is preferably from 1:4 to 1:1. The medium essentially comprises compounds selected from the group comprising the general formulae I to XII.
The media available from the prior art do not make it possible to achieve these advantages whilst simultaneously retaining the other parameters.
For supertwisted (STN) cells, media are desired which facilitate a greater multiplexing ability and/or lower threshold voltages and/or broader nematic phase ranges (in particular at low temperatures). To this end, a further extension of the parameter latitude available (clearing point, smectic-nematic transition or melting point, viscosity, dielectric values, elastic values) is urgently desired.